Electrolyte baths, which are used for electroplating and electroless plating of metals and alloys, typically contain a large number of chemical components. The concentration of many of these chemical components affects the plating rate and the properties of the plated materials. Furthermore, the plating bath composition changes over time due to, for example, chemical and electrochemical degradation of key bath components. Continuous or frequent monitoring of the electrolyte bath compositions is key to maintaining the bath components in proper proportion. The monitoring of the plating bath composition may be coupled with a control procedure that automatically or manually adjusts bath composition.
For convenience in description herein, the terms “plating,” “electroplating” and “electroless plating,” are used interchangeably with the equivalent terms “deposition,” “electrodeposition” and “electroless deposition,” respectively, as is common in the art. In addition to inorganic acids or salts, most commercial plating baths contain an extensive combination of organic additives that are present in very small concentrations. These organic additives (e.g., organic levelers, suppressors, inhibitors, accelerators, superfilling agents, surfactants, wetting agents, etc.) can have a dramatic effect on deposit properties and also influence the plating rate. Unfortunately, bath additive concentrations can change or age with time. This aging of concentrations requires that the bath used in a plating process should be continually or frequently monitored. Inorganic additives such as chloride ions also may be present in very small concentrations.
For electroplating processes, the concentration of organic additives in the plating bath may be the most difficult component to monitor and control. Furthermore, one or more additives are typically present at very low concentrations. In the case of copper plating processes, electrochemical methods, which indirectly measure the impact of the additives, are used for bath monitoring. To determine the concentration of individual additives, isolation methods have been developed for mixing plating bath samples with other fluids, which allow the influence of each additive to be isolated and measured individually. Implementation of these methods requires bulky equipment and produces significant amounts of waste per measurement. Furthermore, measurement times are unnecessarily long. For electroless plating processes, in addition to monitoring bath composition, other monitors are required for measuring plating rates.
Electrochemical measurement methodologies are commonly used for monitoring organic additives in electroplating baths. At least two methodologies based on cyclic voltammetric stripping (CVS) and pulsed cyclic galvanostatic analysis (PCGA) have had recent commercial success for monitoring copper electrodeposition processes in semiconductor manufacturing. (See e.g., commercial chemical monitoring systems sold by ECI Technology, Inc., 60 Gordon Drive, Totowa, N.J. 07512, and ATMI Inc., Danbury, Conn. 06810).
Successful application of electrochemical methods requires reproducible electrode surfaces and suitable electronics to allow for either two or three-electrode measurements in combination with an electrochemical cell. The suitable electronics typically may include a potentiostat, a galvanostat, or a power supply, which may further be connected to appropriate auxiliary equipment such as multimeters, voltmeters, coulometers, etc. A mechanism of recording the electrochemical measurements is also required. Typically this is achieved by interfacing a computer to the electronics. In a low-cost embodiment of the present invention, the recording device may be an analog readout of the measurements by the suitable electronics. Additionally, reproducible and controllable fluid flow within the electrochemical cell is required. A known fluid flow method exploits a rotating disk electrode for creating reproducible flow conditions. However, the disadvantage of a rotating disk electrode is the requirement of a relatively large volume of fluid in the electrochemical cell, generating significant waste since the fluid can generally not be re-used.
Since, in general, an electrochemical measurement is affected by all chemical constituents present, the electrochemical measurement is performed not only on the plating-bath sample but on combinations of the plating bath sample with one or more additional (second, third, etc.) fluids. For an acid-copper electrolyte the second fluid typically contains sulfuric acid and cupric sulfate at the same nominal concentration of the plating bath. The second fluid may also contain one or more of the previously mentioned plating additives. By repeating and recording the electrochemical measurement on a variety of combinations of the plating bath sample and different additional (e.g., second) fluids, methods have been created to determine the concentration of each additive in a bath. In conventional bath monitoring, the series of processes comprising combining fluids and repeating and recording electrochemical measurements not only generates excessive waste but is time consuming, requiring for a complete acid-copper bath at least one hour. Furthermore, because of the volume requirements of the various fluids, the plating bath monitor is large, occupying very valuable real estate in a fabrication facility.
Electrolyte baths are also used for electroetching or chemical etching of workpieces or parts. The etchants used may, for example, contain one or more chemical components or reactants. The rate of etching and quality of the surface of the part undergoing etching depends on the concentration of the etchant components present in the bath. An etchant after a period of usage in a bath will also contain additional chemical components, i.e., products formed by reaction of the reactants and the etched materials. These reaction products in some cases can lead to poisoning of the etchant bath. Furthermore, the original etching bath composition can change in time, for example, due to chemical and electrochemical degradation of key bath components. Therefore, it is desirable to monitor the etch bath composition with time, for example, in industrial manufacturing processes. In addition, it may be desirable to monitor a fresh etchant or electroplating bath as delivered from the manufacturer to obtain a reference level for certain constituents for future comparisons.
Methods of monitoring the etching baths are key to maintaining the bath components in proper proportion. The bath-monitoring method may be coupled with a control procedure that automatically or manually adjusts bath composition. The concentrations of one or more component may be monitored. An individual monitored component may be an original etchant component or a reaction product that is formed during etching of a part. To determine concentration of individual components, which are typically present at very low concentrations, methods have been developed for mixing etching bath samples with other fluids, for example, to isolate the individual component from the influence of other components and for titration.
The etchant bath monitoring methodologies presently used require bulky equipment and produce significant waste per measurement, which makes them environmentally unfriendly. Furthermore, measurement times are long. In general, the monitoring determines the effectiveness of the etchant as constituents change with time during the etching process.
Consideration is now being given to improving systems and methods for monitoring and controlling plating bath and etching bath compositions.